(Not Applicable)
(Not Applicable)
The present invention pertains generally to steam desuperheaters and, more particularly, to a nozzle assembly for a steam desuperheater for reducing steam temperature by spraying cooling water into a steam flow.
Many industrial facilities operate with superheated steam that has a higher temperature than its saturation temperature at a given pressure. Because superheated steam can damage turbines or other downstream components, it is necessary to control the temperature of the steam. Desuperheating refers to the process of reducing the temperature of the superheated steam to a lower temperature, permitting operation of the system as intended, ensuring system protection, and correcting for unintentional amounts of superheat.
A steam desuperheater can lower the temperature of superheated steam by spraying cooling water into a flow of superheated steam that is passing through a steam pipe. Once the cooling water is sprayed into the flow of superheated steam, the cooling water mixes with the superheated steam and evaporates, drawing thermal energy from the steam and lowering its temperature. If the cooling water is sprayed into the superheated steam pipe as very fine water droplets or mist, then the mixing of the cooling water with the superheated steam is more uniform through the steam flow. On the other hand, if the cooling water is sprayed into the superheated steam pipe in a streaming pattern, then the evaporation of the cooling water is greatly diminished. In addition, a streaming spray of cooling water will pass through the superheated steam flow and impinge on the opposite side of the steam pipe, resulting in water buildup. This water buildup can cause erosion and thermal stresses in the steam pipe that may lead to structural failure. However, if the surface area of the cooling water spray that is exposed to the superheated steam is large, then the effectiveness of the evaporation is greatly increased.
In addition, the mixing of the cooling water with the superheated steam can be enhanced by spraying the cooling water into the steam pipe in a uniform geometrical flow pattern such that the effects of the cooling water are uniformly distributed throughout the steam flow. Likewise, a non-uniform spray pattern of cooling water will result in an uneven and poorly controlled temperature reduction throughout the flow of the superheated steam. Furthermore, the inability of the cooling water spray to efficiently evaporate in the superheated steam flow may also result in an accumulation of cooling water within the steam pipe. The accumulation of this cooling water will eventually evaporate in a non-uniform heat exchange between the water and the superheated steam, resulting in a poorly controlled temperature reduction.
Various desuperheater devices have been developed to overcome these problems. One such prior art desuperheater device attempts to avoid these problems by spraying cooling water into the steam pipe at an angle to avoid impinging the walls of the steam pipe. However, the construction of this device is complex with many parts such that the device has a high construction cost. Another prior art desuperheater device utilizes a spray tube positioned in the center of the steam pipe with multiple nozzles and a moving plug or slide member uncovering an increasing number of nozzles. Each of the nozzles is in fluid communication with a cooling water source. Although this desuperheater device may eliminate the impingement of the cooling water spray on the steam pipe walls, such a device is necessarily complex, costly to manufacture and install and requires a high degree of maintenance after installation.
As can be seen, there exists a need in the art for a desuperheater device for spraying cooling water into flow of superheated steam that is of simple construction with relatively few components requiring low maintenance. Furthermore, there exists a need in the art for a desuperheater device capable of spraying cooling water in a fine mist with very small droplets for more effective evaporation. Finally, there exists a need in the art for a desuperheater device capable of spraying cooling water in a geometrically uniform flow pattern for more even mixing throughout the flow of superheated steam.
The present invention specifically addresses and alleviates the above referenced deficiencies associated with steam desuperheaters. More particularly, the present invention is an improved nozzle assembly for a steam desuperheater that is configured to spray cooling water into a flow of superheated steam in a geometrically uniform spray pattern. The nozzle assembly has a forward and an aft end and comprises a nozzle housing, a valve element, and at least one valve spring. The nozzle housing has a hollow configuration open at the forward and aft ends. Importantly, a nozzle barrel disposed within the nozzle housing has an open annular barrel chamber disposed near the forward end of the nozzle to minimize or eliminate a tendency for the cooling water to enter the superheated steam in a streaming spray. The barrel chamber housing may further be configured with a maximum of three flow passages in the barrel chamber to provide a flow of the cooling water from the aft to the forward end of the nozzle housing. By limiting the number of flow passages to three, the tendency for the cooling water to exit the nozzle assembly in a streaming spray is further reduced.
The valve element is slidable within the nozzle barrel such that when the valve element is displaced away from the forward end of the nozzle housing, a flow orifice is created through which the cooling water may flow. The valve element has a valve head configured in a truncated conical shape for imparting a conical spray pattern to the cooling water as it exits the nozzle assembly. The valve element regulates the flow of cooling water through the flow orifice. The valve spring is operatively engaged to the valve element and biases the valve element against the forward end of the nozzle housing to initially seal the nozzle assembly in a closed position. A control valve of the superheater device increases the fluid pressure within the nozzle housing which in turn opens the nozzle assembly by forcing the valve head away from the nozzle housing, allowing for the flow of cooling water into the superheated steam.
A layer of screen mesh may be disposed at the forward end of the nozzle barrel. The screen mesh introduces a fine turbulence into the flow of cooling water through the nozzle barrel, thereby assisting in the formation of droplets. A swirl barrel may be substituted for the nozzle barrel in the nozzle housing. The swirl barrel imparts a spiral motion to the cooling water prior to discharge out of the flow orifice into the superheated steam flow so that the cooling water enters the steam flow in a swirling cone-shaped mist. The geometrically uniform mist pattern ensures a thorough and uniform mixing of the cooling water with the steam flow. The uniform mist pattern also maximizes the surface area of the cooling water spray and thus optimizes the desuperheating effect per unit mass of cooling water. A fracture ring may also be disposed at the forward end of the nozzle housing to aid in the reduction of the water droplet size of the cooling water. The fracture ring is positioned forward of the nozzle housing such that the flow of cooling water spray exiting the nozzle housing impacts the fracture ring, further reducing the droplet size.